Previous work in our laboratory has established that hamster embryo fibroblasts transformed by cytomegalovirus (CMV) express virus specific membrane antigen(s) and that spleen cells from hamsters bearing syngenic tumor transplants or immunized with CMV are specifically cytotoxic for the transformed cells. Spleen cells from herpes simplex virus or SV40 immunized animals, or heterologous tumor bearers are not reactive. Despite the in vitro evidence of cell-mediated immunity the isografts grow progressively. In fact, in virus immunized hosts, tumor growth may actually be enhanced. We propose to extend these investigations of the host's response to CMV tumor antigens. In vivo and in vitro experiments will be carried out to determine whether solubilized virus specific membrane antigen and its antibody may circumvent immunosurveillance and, if so, the mechanism(s) involved. Serological studies will focus on developing sensitive procedures for detecting CMV antigens within the transformed cells. The reagents and methods developed from the foregoing studies will be evaluated for their usefulness in screening human malignant cells for CMV specific antigens. Thus, the experiments proposed should help to clarify the complex interactions between the host and its tumor, and in addition, provide a basis to begin evaluating CMV's oncogenic potential in man.